SONAR (Sound Navigation And Ranging) is the generic name of the technology that is used to locate objects underwater. SONAR is used in marine, geological, and biological research, undersea mapping and navigation, and various commercial and military applications. Among its many uses, SONAR can accurately estimate the velocity of a sea-faring vessel.
It is known to apply signal correlation to SONAR technology to measure velocity. See, for example, U.S. Pat. No. 4,244,026 to Dickey and U.S. Pat. No. 5,315,562 to Bradley et al. These systems typically include a sonar source and multiple receivers (i.e., hydrophones), which have a known separation. The SONAR source directs sonic pulses towards the ocean floor, and the receivers detect echoes of those pulses. The velocity of the vessel is then calculated based upon the distance traveled by the vessel between the transmission and reception of a first pulse and a second pulse.
As discussed further below, correlation SONARS rely on selecting a best or maximum “correlation” either between hydrophones or pulses, for the determination of velocity. Maximum correlation occurs when the ray path of an initial SONAR transmission (from the transmitter to the ocean floor, etc., and back to a receiver) of a first detected pulse is equal to the ray path of a second SONAR transmission.
Correlation SONAR systems can be water or ground referenced, and Spatial or Temporal based. A water-referenced correlation SONAR uses echoes reflected from the water beneath a vessel, whereas a ground-referenced correlation SONAR uses echoes reflected from the ocean bottom. A correlation SONAR can also be both ground and water based in the sense of having both ground- and water-referenced modes of operation.
Spatial correlation SONAR calculates the velocity of a vessel by transmitting two or more pulses towards the ocean bottom, detecting echoes of the pulses on a typically planar two-dimensional array of hydrophones, determining which two hydrophones in the array correlate the best, and dividing the distance between those hydrophones by twice the time differential between the pulses. Peak correlation might take place between hydrophones, in which case an interpolation scheme is used to determine where the peak occurs.
Temporal correlation SONAR determines velocity by transmitting multiple pulses toward the ocean bottom and detecting echoes of the pulses at a hydrophone array. For a given pair of hydrophones, the system determines which two pulses correlate the best, and calculates velocity by dividing the fixed distance between the hydrophones by twice the time differential between the two correlated pulses.
Correlation SONAR systems have a reliability issue that has hitherto not been solved. In particular, serious performance degradation can occur or the system can be rendered inoperable in the event that there are failures in multiple hydrophone channels, where a “hydrophone channel” is defined herein as including a hydrophone and all associated cabling, signal routing, and processing of that hydrophone's output.